1. Field of the Invention
This invention relates in general to apparatus for modifying the time base of signals in a train thereof; more particularly, the invention is concerned with apparatus for correcting a time base instability in TV-type signals.
2. Description Relative to the Prior Art
While the invention is not so restricted, its utility as a flutter compensator in a video tape recorder dramatically indicates its significance. The playback of a taped video recording requires faithful replay of recorded signals without causing such signals to shift unwantedly and continuously in frequency. Unwanted frequency modulation of playback video signals can, among other things, cause (1) the sync information content of the video signal to become unstable, and (2) the color information content of the video to become, at best, faulty, i.e., color information may not be detectable at all, since unstable playback of recorded color burst information within the video signal may cause the color content of the video signal to be lost entirely.
The degree of unwanted frequency shift during playback of the video signal is measured in terms of "percent flutter", it being recognized that even fractional percent flutter will prevent the AFC circuits of most TV's from tracking the fluttering sync signal, resulting in the tearing apart of the visual display that corresponds to the playback signal. Percent flutter may be defined as 100 times a change in frequency divided by a nominal frequency, the nominal frequency in the present case being 15750 Hz.
To assure against instantaneous time base instability of played back TV signals which are derived from a video tape recorder, present practice is to employ extremely close tolerances in the precision parts of the recorder and to complement such tolerances with various servo controls which actively prevent flutter from occurring during playback. Such practices greatly add to the cost of manufacturing video tape recorders and, indeed, have prevented video recorders from being cost-wise within reach of many who would otherwise want them. This being the case, various proposals have been made to utilize variable delay lines, e.g., U.S. Pat. Nos. 2,960,568; 3,238,300; and 3,580,991, as a way to nullify flutter within playback signals: flutter within the played-back signals is detected; the "flutter" signal is then used to vary the delay of a delay line through which the fluttering signal is passed, thereby causing the output of the delay line to be, hopefully, a flutter-free equivalent of the fluttering input to the delay line.
Copending U.S. Pat. application Ser. No. 432,363, filed Jan. 10, 1974, discloses a departure from the above-noted delay line concept, having, at the heart thereof, and analog shift register. Basically, copending Ser. No. 432,363 teaches (1) the clocking (sampling) of a fluttering signal into an analog shift register at a rate dependent on the degree of flutter, and (2) the clocking of such signal samples out of such analog shift register at a desired rate. If the desired rate is constant, flutter is nullified. On the other hand, the clock-out rate may be variable, whereby the signal output of the analog shift register may be modified according to choice.
In utilizing the teaching of Ser. No. 432,363 to process signals such as TV-type signals, i.e., signals which are continuous and which have a nominal periodicity, plural analog shift registers, say two, are used and so cooperate that while one TV signal is clocked into one register at a flutter-dependent rate, the signal within the other register is clocked out of such register at a known, but stable, rate, and vice versa.
As is known, analog shift registers may take a variety of forms such, for example, as charge coupled and charge injection devices, various serially disposed sample-and-hold circuits, and even A/D and D/A converters which functionally sandwich digital shift registers.
Recognizing that time base stability in the lead portion of, say, a TV-type signal is of more consequence than time base stability in the mid or trailing parts of such a signal (i.e., the lead signal portion contains the color burst information and also critically affects horizontal synchronization of a TV set, whereas time base shift in mid or trailing signal portions causes the mere shift of visual scene content -- which will frequently occur off the face of the TV, and/or be relatively unnoticeable, anyway), copending application Ser. No. 477,583, filed June 10, 1974, taught an invention which obviated the need, as in Ser. No. 432,363, for a variable frequency clock to load an analog shift register at a flutter dependent rate and, instead employed a fixed clock in combination with variable loading of an analog shift register(s). In a somewhat broad sense, then, the invention of Ser. No. 477,583 proposed the following as a way to nullify the effects of time base instability: Successively clock periodically occurring signals which may be subject to time base instability, and which have a nominal duration and periodicity, into an analog shift register by means of a fixed clock having a frequency which is just sufficient to shift the first sample of each signal to the output stage of the analog shift register within the duration of each such signal whereby, for each periodic signal, samples will appear in a number of the stages of the analog shift register in proportion to the degree that each such signal is stretched time-wise. Then, in response to a reference pulse train having time base stability, unload the analog shift register of its samples at a clock rate which will unload all stages of the analog shift register within the nominal duration, thereby causing (1) the lead part of all of the periodically occurring signals to have time base stability, and (2) those periodic signals, which before processing were stretched, to have the nominal duration after processing, all other post-processed signals having durations which are less than the nominal duration by tolerable amounts.
In a preferred embodiment of the invention of Ser. No. 477,583, as might be employed to remove flutter from a TV signal train derived from the playback of magnetic tape or the like, three analog shift registers, e.g., charge coupled devices (CCDs) are employed to store, respectively, samples of every third video line signal (which signals shall be taken to include both video and sync information) in a signal train: The first video signal is clocked into the first CCD at a first clock rate; then the second video signal is clocked into the second CCD at the first clock rate, and while such occurs, the signal in the first CCD is clocked out of the first CCD at a second rate which will assuredly clear all stages of the first CCD during the time of an unfluttered video signal; then the third video signal is clocked into the third CCD at the first clock rate, and while such occurs, the signal in the second CCD is clocked out of the second CCD at the second clock rate; then the signal in the third CCD is clocked out of the third CCD at the second clock rate, and while this occurs, the fourth video signal is clocked into the first CCD at the first clock rate, and so on. By so selecting the first clock rate that a "worst case" stretched video signal fully loads all stages of a CCD with signal samples within the duration of the worst case signal, each CCD is more or less, starting with its output stage, periodically loaded with signal samples, the number of which is in proportion to the degree of stretch that the corresponding video signal experiences. As the CCDs are successively unloaded at the horizontal sync rate, there is no flutter of the critical sync portions of the video signals clocked out of the CCDs; and whatever flutter remains within the clocked-out signals virtually unnoticeably occurs in the visual part of, and more particularly off the right-hand side of, the TV picture.